The present invention relates to an optical cross-connect system and an optical transmission system, and more particularly to an optical cross-connect system and an optical transmission system which allow a line or a path for a high-speed optical signal to be switched with the signal light itself unconverted.
Recent advances in optical transmission technology are rapidly making it possible to increase capacity on a network up to a high-capacity level. This also means, however, that when a failure occurs, the damage can not help being expanded accordingly. This situation makes it urgent to construct a network with a high reliability. In prior arts, a cross-connect system played a role of specifying a by-pass route around a portion at which the failure occurs. However, the cross-connect system, which necessitated an electro/optical conversion and a multiplexing division into low-speed signals, found it difficult to process such a high-capacity optical signal. In view of the circumstances, developed at many places is an optical cross-connect system which allows the high-speed optical signal to be switched with the signal light itself unconverted, i.e. without performing the electro/optical conversion thereof. FIG. 1 shows a configuration embodiment of a network which introduced into is an optical cross-connect system and has a function of automatically avoiding a failure. In FIG. 1A, which indicates a normal state, line terminals 1 are connected with each other through a working optical fiber 3. On the other hand, FIG. 1B indicates a state in which there takes place a failure in the working optical fiber 3 between a terminal station A and a terminal station B. More concretely, illustrated schematically in FIG. 1B is a situation in which an optical cross-connect system 2, through a protecting optical fiber 4, forms a by-pass route by way of a terminal station C, thus making it possible to restore the transmission path.
Mentioned as a literature of an optical cross-connect system having such a failure-restoring function is "A Novel Optical Cross-connect System for Hitless Optical Network Reconfiguration", a presentation No. SB-8-1 at an autumn general conference held in 1993 under Institute of Electronics, Information and Communication Engineers. Proposed and studied in the report are a 64.times.64 switch matrix and an optical cross-connect system using it. The 64.times.64 switch matrix is constituted by employing a 8.times.8 switch matrix as the building block thereof and connecting the 8.times.8 switch matrices in a three-stage link connection manner. As shown in this embodiment, in related-art optical cross-connect systems, a general method for embodying the high-capacity was as follows: A strictly non-blocking switch matrix is employed as the fundamental building block and then performing a link connection of the matrices, thereby embodying the high-capacity. Here, the switch matrix means a switch configuration in such a broader meaning as to make it possible to switch and connect a plurality of inputs and a plurality of outputs, and includes configurations such as a tree type switch configuration.
As shown in FIG. 1, the optical cross-connect system 2 is provided at each node on a network and has a function of changing a connection between a line terminal 1 and a transmission path, i.e. the optical fiber 3 or the optical fiber 4. Illustrated in FIG. 2 is a basic system configuration of an optical cross-connect system in the case where M units of line terminals within a node are connected with an optical switch unit 11 through 2M units of optical fibers 13, and the number of working optical fibers 14 and that of protecting optical fibers 15 are set to be 2M and 2R, respectively. A monitor unit 12 detects failures in the fibers, and the optical switch unit 11, which a control unit 10 controls, performs switching of connections. Optical signals are launched into or out of the optical switch unit 11, i.e. a main unit in the optical cross-connect system, from both the line terminal side and the transmission path side. When organizing the optical signals in accordance with the directions thereof, it has been found that the result is summarized as an optical switch matrix 18. The optical switch matrix 18, as shown in FIG. 2B, is a square matrix having 2M+R units of input ports and 2M+R units of output ports, i.e. a switch matrix in which the number of inputs is equal to that of outputs.
Generally speaking, -the optical fibers 14 or the optical fibers 15 are installed as a cable produced by bundling about 24 to 47 units of the optical fibers in total, and connected with each node are cables originating from a plurality of neighboring nodes. Accordingly, the number of the optical fibers for each node extends to a scale of 200 to 300. This requires that the optical crossconnect system, which operates with these optical fibers, also have a high capacity corresponding thereto. The biggest problem in embodying such a high-capacity optical cross-connect system lies in making the optical switch unit 11, i.e. the main unit in the optical cross-connect system, into a large scale switch matrix.
Combination of a plurality of optical switch devices makes it possible to embody such a large scale switch matrix. It is desirable that scale of each optical switch device itself is large, i.e. the degree of integration thereof is high. The degree of integration of an optical device, however, is generally so much lower compared with that of an electronic device. For example, as described in the related art, it is close to a limit of the present-day technology to integrate the 8.times.8 switch matrices on a single chip. Also, structures of optical switch devices employed in currently embodied integrated-type switch devices (such as 4.times.4, and 8.times.8) are generally inferior to those of single-type switch devices (such as 1.times.2, and 2.times.2) in the fundamental characteristics such as isolation at the time of switching and the insertion loss. This inevitably gives rise to a deterioration in the optical signal quality at the time of switching, thus making it difficult to apply to the high-speed signal the large scale switch matrix which is embodied using the integrated-type switch devices.
Meanwhile, when constituting the switch matrix with the use of the single-type switch devices of excellent fundamental performance, in order to make the capacity of the matrix higher, there are needed conditions such as an increase in the number of the members and a processing of optical wiring among the devices. This inevitably brings about a rapid increase in the system scale, thus making it difficult to embody a switch matrix on a scale of practical use. Moreover, even when employing the high-performance single-type switch devices, optical amplifiers 16 and 17 in FIG. 2 become essential owing to compensation for the insertion loss. This results in problems such as a deterioration in the optical S/N ratio of the signal due to an occurrence of optical noise at the time of amplifying the signal light, or an increase in the system cost due to an increase in the number of the amplifiers.
As described above, in trying to embody an optical cross-connect system of practical use, it has become the most important problem to develop a large scale switch matrix which when applied to a high-speed optical signal, gives rise to a less deterioration in the signal quality.